This invention concerns an oxygen sensor comprising a sensor device for detecting an oxygen density and a method of manufacturing the sensor device.
When a oxygen sensor comprising a sensor device is exposed as an air/fuel ratio sensor to an exhaust gas upon using lead-containing fuels, electrodes are contaminated with lead, phosphorus, silicon or the like and degraded with lapse of time to no more provide sufficient electromotive force. Sensors coping with the problem of deterioration for the durability of electrodes are disclosed, for example, in Japanese Patent Publication No. 90176/1994 and Japanese Laid-Open No. 113480/1997. However, a sensor capable of completely preventing contamination to electrodes in a low temperature region which is most likely to undergo the effect of lead or the like contained in exhaust gases has not yet been obtained.
This invention intends to provide an oxygen sensor having a sensor device capable of preventing an electrode from contamination by lead or the like and having excellent durability even in contact with exhaust gases at a relatively low temperature, as well as a method of manufacturing such a sensor device.
The oxygen sensor of the present invention is an oxygen sensor comprising a sensor device including a detection electrode, an electrode protection layer formed on the surface of the detection electrode and a contamination preventive layer formed on the surface of the electrode protection layer, wherein the contamination preventive layer comprises a composite powder having a ceramic powder having a large grain size (referred to hereinafter as coarse powder) covered therearound with a ceramic powder having a small grain size (referred to hereinafter as fine powder), and hollows not filled with the fine powder are scattered in gaps among such composite powders.
By forming the contamination preventive layer as described above, contamination materials are trapped by the fine powder and prevented from reaching the electrode in the oxygen sensor, thus preventing a deterioration by contamination in the performance of the oxygen sensor. On the other hand, the fine powder is carried on the coarse powder, thus preventing the problem of removal of a contamination preventive layer from a sensor device surface such as a layer composed exclusively of the fine powder upon high heat setting during continuous use at high temperatures. Further, the fine powder is carried so as to cover the surface of the coarse powder, while suitable hollows having a similar size to that of the coarse powder are formed in gaps among the coarse powders, so that the gaps among the coarse powders are not completely filled with the fine powder, and therefore, the contamination preventive layer cannot be clogged even upon accumulation of comtamination materials thereon, to prevent a reduction in the response of the sensor.
The grain size distribution of primary particles of the ceramic powder constituting the contamination preventive layer as described above has at least two peaks, and if the peak on the side of the smallest grain size is 10 xcexcm or less and the peak on the side of the largest grain size is 0.1 xcexcm or more, a desirable contamination preventive layer having a high contamination preventing effect is given.
In this case, the peak on the side of the small grain size is preferably at 1 xcexcm or less and can be at 0.05 xcexcm or less and, particularly, at 0.01 xcexcm or less. Further, the peak on the side of the large grain size is preferably at 1 xcexcm or more and can be, particularly, 10 xcexcm or more.
For good biting of the coarse particle, the electrode protection layer under the contamination preventive layer is formed preferably by spray coating.
The xe2x80x9cceramic powderxe2x80x9d contained in the xe2x80x9ccontamination preventive layerxe2x80x9d is preferably selected from the powder of oxide which is chemically stable in exhaust gases at high temperature such as titania, alumina, silica, and composite oxide containing aluminum atoms such as spinel and mullite. Powder other than the oxide may also be used so long as it is chemically stable. In this case, two or more kinds of ceramic powders of different compositions may be mixed. When the ceramic powder in one composition is a fine powder while the ceramic powder in the other composition is a coarse powder, the degree of freedom is extended in the selection of the powder to facilitate the provision of the powder having a desired grain size distribution, and such ceramic powders can be used conveniently by selecting a ceramic powder having a high contamination-preventing effect as the fine powder and a ceramic powder having high-temperature durability as the coarse powder.
The two or more kinds of ceramic powders of different compositions, preferably, contain a titania powder having a peak in the grain size distribution at 1 xcexcm or less and a ceramic powder other than titania having a peak in the grain size distribution at 10 xcexcm or more.
Titania is considered to be excellent in the ability to adsorb contamination materials. In particular, anatase type titania can be used for easily providing a powder having a small grain diameter and a high contamination preventing effect.
Preferred ceramic powder other than titania is, particularly, less heat shrinkable ceramic powder such as a composite oxide containing aluminum atoms such as spinel or mullite.
Further, a titania powder having a peak from 0.003 to 0.5 xcexcm is combined particularly preferably with a ceramic powder other than titania having a peak from 15 to 50 xcexcm in order to form suitable gaps in the contamination preventive layer. Such powders can be incorporated to provide a contamination preventive layer which can sufficiently adsorb contamination materials, is not removed by thermal shrinkage from the electrode protection layer and is further superior in durability with less drop in response.
That is, when a powder having a peak of small grain sizes in the range of 1 xcexcm or less, preferably 0.003 to 0.5 xcexcm is used in combination with a powder having a peak of large grain sizes in the range of 10 xcexcm or more, preferably 15 to 50 xcexcm, the contamination preventive layer as shown in FIGS. 1(a) and (b) having suitable hollows with a similar size to that of coarse powder therein is formed from composite powders comprising a large number of particles of the powder with small grain sizes adhering to the surfaces of particles of the powder having large grain sizes, so the contamination preventive layer can sufficiently keep air-permeability, can certainly adsorb contamination materials, and can be rendered highly durable.
As the coarse powder and fine powder, powders identical in the composition but different in the crystal phase can also be selected. It is particularly preferable that an anatase type titania powder is used as the fine powder and a rutile type titania powder as the coarse powder. Both powders are titania powders but are different in the crystal phase, and these are provided as fine and coarse powders having a narrow distribution of grain sizes and are thus suitable for forming a contamination preventive layer excellent in air-permeability. For the contamination preventive effect, the grain diameter in the peak of the grain size distribution of the anatase type titania powder is preferably 0.5 xcexcm or less, more preferably in the range of 0.003 to 0.5 xcexcm. For the contamination preventive effect, the grain diameter in the peak of the grain size distribution of the rutile type titania powder is preferably 1 xcexcm or more, more preferably in the range of 3 to 8 xcexcm. By combination of the anatase type titania powder having a very small grain diameter of about 0.003 to 0.5 xcexcm with the rutile type titania having a larger grain size, the contamination preventive layer excellent in the action of capturing poisoning materials can be formed. Further, by using the ceramic powders having the same composition, the composite powders can be easily formed to provide the contamination preventive layer with a good contamination preventing effect.
When the grain size distribution of the contamination preventive layer in the product is evaluated, the grain size can be read in the field of view of an electronic microscope on one hand, or from a photograph thereof. When the grain size is read from the field of view of the electronic microscope or from its photograph, each of the primary grains that can be confirmed visually is measured for the diameter of circumcircle and determined as a grain size. Measurement for the grain size is conducted for a plurality of primary particles (about 1000) to calculate the grain size distribution. In a case of using oxide powders of different compositions, the grain size can be measured to determine the grain size distribution on the oxide powder of each of the compositions. However, in view of the gist of this invention, when the grain size is measured in a mixed state of the fine grain powder and the coarse grain powder, it is not necessary to measure the grain size distribution on every ceramic powders of different compositions but the grain size distribution may be measured using grain size diameter sampled at random from the contamination preventive layer. As a result, it may suffice that the peak on the side of the small grain size is 1 xcexcm or less and the peak on the side of the large grain size is 0.1 xcexcm or more.
On the other hand, grain size distribution of the fine grain powder is sometimes difficult to measure using a usual scanning type electron microscope or the like and it can be measured in the same manner as described above by using an electron microscope of high resolution power, but it can also be calculated according to the Scheller""s equation by using an X-ray small angle scattering method for measuring the grain size distribution of the powder.
In addition, the grain size distribution can also be measured by a generally utilized method such as a laser beam diffraction method or centrifugal precipitation method. However, it is often difficult to measure the grain size distribution for an identical sample from a fine region to a coarse region by an identical measuring method. In such a case, the grain size distribution in the fine region and the coarse region may be measured by different measuring methods and the grain size distribution in the contamination preventive layer may be identified based on the respective grain size distributions.
The method of manufacturing the sensor device in the oxygen sensor of the present invention comprises kneading one or more kinds of first ceramic powders, one or more kinds of second ceramic powders with a peak of a grain size distribution of primary particles being on the side of a large grain size than the peak of a grain size distribution of primary particles of the first ceramic powder whereupon the difference between the maximum grain diameter of 10% particles on the side of a small grain diameter (referred to hereinafter as 10% grain diameter or d10) and the maximum grain diameter of 90% particles on the side of a small grain diameter (referred to hereinafter as 90% grain diameter or d90) is not more than twice of the grain diameter as the peak value of the grain size distribution, an organic binder and a solvent to prepare a contamination preventive layer-forming paste, coating said contamination preventive layer-forming paste on the surface of the electrode protection layer of an oxygen sensor device to form a coating and subsequently heating and drying said coating thereby forming the contamination preventive layer. According to this method, a powder having a regular grain diameter near to a peak in the distribution of grain sizes is used as the second ceramic powder serving as a coarse powder in the contamination preventive layer, thus facilitating formation of the contamination preventive layer having hollows with a similar size to that of the coarse powder scattered therein. Further, by incorporating an inorganic binder suitably into the contamination preventive layer-forming paste, the fine powder can adhere to the surface of the coarse powder to achieve an excellent contamination preventive layer. Further, the first ceramic powder serving as the fine powder and the second ceramic powder as the coarse powder are preferably highly heat-resistant oxides.
In particular, a titania powder or the like having a specific surface area of 2 to 500 m2/g can be used as the first ceramic powder and an aluminum atom-containing composite oxide powder or the like having a specific surface area of 0.1 to 100 m2/g as the second ceramic powder. Alternatively, an anatase type titania powder having a specific surface area of 2 to 500 m2/g is used as the first ceramic powder and a rutile type titania powder having a specific surface area of 0.1 to 10 m2/g as the second ceramic powder, and from these powders, the contamination preventive layer can also be formed in an analogous manner.
The specific surface area of the first ceramic powder is 2 to 500 m2/g, particularly preferably 5 to 300 m2/g. If the specific surface area is less than 2 m2/g, the resulting contamination preventive layer is deteriorated in both physical capture of contamination materials and reaction therewith, while if the specific surface area is greater than 500 m2/g, the powders easily aggregate and their reactivity with contamination materials becomes too high, and the response of the resulting oxygen sensor gradually changes unfavorably under a high-temperature environment. On the other hand, the specific surface area of the second ceramic powder is 0.1 to 100 m2/g, particularly preferably 0.3 to 10 m2/g. If the specific surface area is less than 0.1 m2/g, a uniform contamination preventive layer having a smooth surface cannot be formed, while it is higher than 100 m2/g, the contamination preventive layer cannot be sufficiently prevented from being aggregated. When the specific surface area of the second ceramic powder is in the range described above, hollows are formed and dispersed in gaps among coarse powders, to provide the contamination preventive layer with good air-permeability.
The specific surface area can be determined according to the BET method. Further, when the specific surface area of the powder is particularly large, it can be measured by using a full automatic surface area measuring apparatus, Model xe2x80x9cMultisoap 12xe2x80x9d manufactured by Yuasa Ionics Co.
The first and second ceramic powders are preferably contained in amounts of at least 15 parts by weight (referred to hereinafter as xe2x80x9cpartsxe2x80x9d) respectively relative to 100 parts of the contamination preventing layer forming paste. If the content of either the first or second ceramic powder in particular of the first ceramic powder is less than 15 parts, contamination materials cannot be sufficiently captured. Further, if the content of the second ceramic powder is less than 15 parts, suitable hollows cannot be formed among the coarse powders in the contamination preventive layer, thus failing to maintain the air-permeability. For forming suitable hollows in the contamination preventive layer, both the first and second ceramic powders are more preferably contained in amounts of 20 to 50 parts respectively. In the ceramic powder, other ceramic powder than each of the powders as the essential constituent factors of this invention can also be mixed, but mixing of the ceramic powder whose grain size distribution for the entire ceramic powder is out of the gist of this invention is not preferred.
Further, the ratio of the mixing amount between the first ceramic powder and the second ceramic powder is not particularly restricted but it is preferred to mix 100 parts of one of them and from 40 to 250 parts, particularly, 80 to 130 parts of the other of them and they may be mixed about in an equal amount. If there is no significant difference in the ratio of the amount of the powders, a homogeneous contamination preventive layer excellent in the function of trapping the contamination substances and having smooth surface can be formed more efficiently.
The xe2x80x9ccontamination preventive layer-forming pastexe2x80x9d is obtained by kneading the ceramic powders, an organic binder and a solvent such as methanol, xylene etc. and a suitable inorganic binder or the like. The coating film formed on the surface of the electrode protection layer is solidified and hardened sufficiently by drying at 100 to 150xc2x0 C. for about 5 to 20 min, after the drying, attaching the sensor device with the dried coating film to a protection tube receptacle, and then heating the sensor device in an assy furnace which is controlled for temperature 300-700xc2x0 C., particularly, to about 400-600xc2x0 C. in a reducing atmosphere for about 20 to 60 min., and then a sensor device provided with a contamination preventive layer having predetermined contamination preventive function and thickness can be formed.
The thickness of the contamination preventive layer shall be 50 to 300 xcexcm, particularly preferably 150 to 250 xcexcm. If its thickness is too small, contamination materials may not be sufficiently captured. On the other hand, if it exceeds 250 xcexcm, it is not preferred since the response of the oxygen sensor obtained is lowered and, further, the contamination preventive layer tends to be exfoliated from the electrode protection layer.